Focal length
focal point When rays of light enter the lens they bend according to the rules of refraction.  The focal length of a lens is the point which parallel light rays come together after bending in the lens. This happens when the lens' focusing control is set at infinity because objects from very far away have rays of light coming in almost parallel to the camera lens.

The focal length of the lens is determined by the amount of curvature of the lens. If the lens were made into a spherical piece of glass, the center of  curvature of the lens is the point which would be the center of the sphere. The focal point is  half of the center of curvature, so the larger the center of curvature, the larger the focal length.		 center of curvature


From the practical, picture-taking standpoint, focal length is important because it determines the degree of image magnification due to the lens. The larger the focal length of the lens the larger the magnification. This can be shown by looking at ray tracing diagrams.

If a lens has a longer focal length, an object will project a larger image onto the film
 If a lens has a shorter focal length, an object placed in the same location will project a smaller image as you can see by comparing the two images formed here in these diagrams.
out
 small image

telephoto



Notice also from these ray tracing diagrams that lenses with larger focal lengths will project the image at a further distance from the lens. This is the reason that "zoom" lenses, or those lenses that magnify the object need to be further away from the film and appear to be longer than a standard lens.

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